In this article are compared traditional OPAC systems, enriched OPAC, social OPAC and social cataloguing systems.the aim is to underline new theoretical trends and to offer a taxonomic outline of such tools, according to the interaction level granted to users and to the chance to manage user's generated contents in the point of view of the application of web 2.0 tendecies to libraries, in the library 2.0. At the end, a brief review of softwares, both open source and not, that seem promising for this future application.